The present invention relates to a hydraulic system for controlling a shift in an automatic transmission.
JP 62-62047 discloses an automatic transmission including a band brake which is engaged in the second and fourth speed ratios, but released in the third speed ratio, and a high clutch which is released in the second speed ratio, but engaged in the third and fourth speed ratios. If a downshift from the fourth speed ratio to the second speed ratio is effected with the third speed ratio being established momentarily, a shock takes place twice, causing unplesant shift feel. In order to prevent the downshift as above from taking place, the known automatic transmission uses two valves, namely a 4-2 relay valve and a 4-2 sequence valve, so as to cause a direct downshift from the fourth speed ratio to the second speed ratio. The 4-2 relay valve delivers a boost pressure when a fourth speed apply chamber of the band brake is pressurized. This boost pressure is supplied via a 2-3 shift valve to the 4-2 sequence valve. The 4-2 sequence valve keeps on delivering the boost pressure until a hydraulic pressure in a third speed release chamber of the band brake drops to a sufficiently low level so as to allow engagement of the brand brake only by a hydraulic pressure acting in a second speed apply chamber. The boost pressure delivered by the 4-2 sequence valve is supplied to a 3-4 shift valve which in turn delivers this boost pressure to the fourth speed apply chamber to keep the band brake engaged when it is in a down position thereof. Thus, the fourth speed ratio is maintained until the band brake is ready to be engaged by the hydraulic pressure in the second apply chamber.
An object of the present invention is to provide a hydraulic system wherein the same function as mentioned above is performed by a reduced number of valves.